Drugs are commonly administered in solid form through pills or capsules that can be orally taken. However, many biological drugs can not be administered this way because of degradation in the gastrointestinal tract and quick elimination by the liver. Another common technique for administration of drugs in liquid form is through injection using a metal hypodermic needle that can cause significant pain and discomfort to patients. A number of physical and chemical techniques including electroporation, laser ablation, ultrasound, thermal, iontophoresis and chemical enhancers have been explored to develop painless transdermal drug delivery techniques. It was found that it's very difficult for the molecules with a molecular weight higher than 500 or diameter larger than 1 nm to penetrate normal human skin. Further studies showed that the key barrier for transdermal delivery of substances is the stratum corneum layer, the outer layer of skin, that is about 4-30 micron thick. Invasive methods to overcome this skin barrier have been used in practice, such as intradermal (ID), intramuscular (IM) or subcutaneous (SC) injection using standard hyperdomic needles and syringes. These methods cause pain and require a skilled professional. In addition, they may cause needle injuries. Similarly, current method of extracting biologic fluids such as blood from patients suffers from the same disadvantages.
In order to improve the skin permeability of the therapeutic agents and other active ingredients, microneedles have been recently developed to disrupt the stratum corneum and facilitate the delivery of the active agents and ingredients to the epidermis. These active substances can then diffuse through the rest of epidermis to the dermis and absorbed by blood vessels and lymphatics there. The substance absorbed can get into circulation system. Thus both topical and system-level delivery of drugs is possible. Since there are no nerves and blood vessels in stratum corneum and epidermis, this is a minimally invasive, painless and blood-free method of drug delivery. An additional advantage of this method, when engineered for topical delivery of vaccines, can lead to enhanced inoculation effect because the epidermis is rich in antigen presenting cells and is a desired target for vaccine delivery.
The prior art reports many devices and methods to overcome the skin barriers. For example, U.S. Pat. Nos. 5,855,801 and 5,928,207 assigned to The Regents of the University of California taught a microneedle fabrication method similar to IC compatible neural recording arrays. The disclosed microneedle arrays are typically linear array as they are in the plane of the silicon substrate surface. Microneedles have been also fabricated by heating the glass tube and lengthening the heated part till the diameter of the tip is reduced to the desired range. It's in general very difficult to control the size of the needle shaft and the tip this way although biologists are still using this method to produce microneedles that can inject or withdraw substances from a single cell.
U.S. Pat. No. 6,503,231 by Prausnitz et al discloses a method for making out-of-the-plane porous or hollow microneedles. It either involves porous silicon formed by anodization of silicon or deals with sacrificial molds or selective removal of substrate materials to form fluidic conduits. U.S. Pat. No. 6,511,463 by JDS Uniphase Corp. also taught a method to fabricate the same. U.S. Pat. No. 6,558,361 assigned to Nanopass Ltd. taught a method for the manufacture of hollow microneedle arrays by removing a selective area of substrate material. U.S. Pat. No. 6,603,987 assigned to Bayer Corp. also disclosed a method to make hollow microneedle patch. All these methods are trying to perform certain functions of the current hyperdomic needles and create a miniaturized analog to perform drug delivery or extract body fluids without causing pain and discomfort.
More recently, U.S. 2004/0241965 discloses a method of making high aspect ratio electrode arrays comprised of solid metals. It involves the preparation of porous microchannel glass template, electrodeposition of metals in the microchannels, and final preparation of electrode array following electrodeposition. The body of microelectrode is formed by electrodeposition method similar to those used in forming nanowires.
The prior methods to make microneedles, whether they are in-the-plane or out-of-the-plane from the substrate material, are cumbersome and expensive. The hollow microneedle arrays, while their sizes are scaled down from conventional needles, are especially expensive to make because of complexity in fabrication process. Their mechanical integrity also suffers as their sizes become smaller. Accordingly, a continuing need exists for an improved low cost, disposable transdermal delivery device for effective through skin delivery of substances in a controlled manner.